The present invention relates to an expandable structure and, more particularly, to a structure such as an expandable antenna which is mounted on a large-sized artificial satellite or a large-scale space structure, together with a method of deploying such an expandable structure.
Expandable antennas for use in space, for example, those mounted on artificial satellites which are launched into space have heretofore been known. One type of conventional expandable antenna is disclosed, for example, in Japanese Patent Application No. 59-28704 (1984). The structure of this type of expandable antenna is shown in FIGS. 4 to 7, in which FIGS. 4 and 5 are front and side views, respectively, of the antenna which is in a packaged state, and FIGS. 6 and 7 are front and side views, respectively, of the antenna which is in an unfolded state.
Referring to the figures, each 180-degree hinge 1a incorporates a spring, for example, a spiral spring, which provides driving force required to unfold the antenna. The hinge 1a has a latch which locks it in the unfolded position when the angle of the hinge 1a reaches 180 degrees. Each 135-degree hinge 1b has an arrangement similar to that of the 180-degree hinges 1a. More specifically, when the angle of the hinge 1b reaches 135 degrees, it is locked in this unfolded position. These hinges 1a and 1b are arranged such that frame members 2a and 2b can be unfolded under low-friction conditions.
The frame members 2a and 2b are connected together at their ends by means of the hinges 1a and 1b to thereby form a hoop-shaped antenna as a whole. In this arrangement, the frame members 2a are not expandable but only the frame members 2b are expandable. The frame members 2a and 2b are defined by tubular members.
A multiplicity of support wires 3 the tension of which is adjustable are secured to the frame members 2a, 2b and the hinges 1a, 1b at predetermined spacings so that a mesh-like flexible antenna member 4 is supported at its peripheral edge by these support wires 3. As shown in FIG. 7, two, upper and lower, flexible antenna members 4 are employed and these two antenna members 4 are independently retained by the respective support wires 3 and connected together through connecting wires 5 in such a manner that the antenna members 4 are pulled toward each other. The connecting wires 5 have different lengths such that the wire length gradually decreases from the peripheral edge of the antenna toward the center, thereby enabling the flexible antenna members 4 to constitute a spherical parabolic antenna. By forming at least one of the flexible antenna members 4 from a material having electrical conductivity (i.e., wave reflection properties), the antenna member 4 is enabled to function as an antenna.
The operation of the above-described conventional expandable antenna will next be explained. After the expandable antenna has been launched into space, the frame members 2a and 2b which are in the packaged state shown in FIGS. 4 and 5 begins to unfold, and the two flexible antenna members 4 which have been folded between the frame members 2a and 2b also begins to expand gradually as the frame members 2a and 2b unfold.
In the unfolding operation, the 180-degree hinges 1a which have been folded inside the frame members 2a and 2b in the packaged state are locked by the incorporated latches, respectively, when the hinge angle reaches 180 degrees, and the 135-degree hinges 1b which have been folded at the outer side of the packaged antenna are similarly locked when the hinge angle reaches 135 degrees. When all the hinges 1a and 1b are locked, a substantially octagonal hoop-shaped antenna such as that shown in FIG. 6 is formed. In addition, a predetermined level of tension is applied to the two flexible antenna members 4 through the support wires 3 and the connecting wires 5. Thus, spherical parabolic surfaces are formed.
The above-described conventional expandable antenna for use in space suffers, however, from the following problems. Since parabolic surfaces are defined by the flexible antenna members 4 which are stretched inside the frame members 2a and 2b, it is necessary in order to ensure the required mirror surface accuracy to increase the number of support and connecting wires 3 and 5 and adjust the level of tension applied thereby, and this inevitably limits the size of the parabolic surfaces. Thus, it has heretofore been impossible to form an expandable antenna having a large aperture.
Further, when the frame members 2a and 2b are in a folded state, the axes of the two leaves of each of the hinges 1a and 1b extend parallel to the plane within which the antenna is unfolded. Therefore, the frame members 2a and 2b are likely to interfere with each other during an unfolding operation and the space required for the folded antenna is disadvantageously large.